Automatic signal level control



Nei di Dec. 7, 1948.

AToRNEY at the multiplier input.

Patented Dec. 7, 1948 AUTODITIC SIGNAL LEVEL CONTROL Robert W. Sanders, Fort Wayne, Ind., assignor,

by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application November 18, 1943, Serial No. 510,753

3 Claims.

particularly to the automatic gain controlof an electron multiplier type of amplifier.

It is customary to employ multistage static electron multipliers to amplify many different types of signals. The manner in which the electron multipliers are connected and operated depends upon the particular use to which they are put. Regardless of the type of signal to be amplified or the faithfulness with which the signal amplification is to be effected, a multistage multiplier is essentially a device which is operated by impressing positive potentials of increasing values to successive ones of the secondary emissive multiplier electrodes. In many cases it is desirable to maintain a substantially uniform potential diierence between successive multiplier electrodes. This type of operation may be achieved in one way by the use of independent sources of potential for each pair of electrodes. In this manner the successivedifferences of potential between any two electrodes is not materially affected by the electron current flow between these two electrodes or by the current conduction in any other part of the multiplier. Thus, the overall multiplication ratio of the device remains substantially constant throughout a considerablev range of variation of the electron concentration at the multiplier input. The voltages developed in the output circuit of the multiplier then are-substantially directly proportional to the primary electron concentrations Such a device is said to have a linear operating characteristic.

There are, however, some uses for an electron multiplier where linearity between input and output is .unnecessary .or even undesirable. One such application of an` electron multiplier is presented frequently in a television system where the multiplier-is to be used in conjunction with a camera tube of the dissector type. When television apparatus of such a character is to be used in a studio or similar location it is a comparatively simple matter to control the subject illumination in a manner suitable to eiect the generation of v video signal voltages representing light and shade ment because of cloud movements, haze conditions and the like. If, in such a: situation .the entire television system, including .the reproduc- (Cl. Z50-41.5).

, 2 ing apparatus. is adjusted for relatively weak light conditions and there is a sudden increase in the subject illumination intensity, it is likely that some part of the system may become so greatly overloaded as to deleteriously affect the reproduced picture. Insuch installations it then becomes necessary to monitor the television signals and to eiect the desired control of the signal voltages to provide a suitable light level in the imlage reproduced under the control of such signa s.

If a multiplier having a linear operating characteristic is employed to amplify the generated video signals, the peak-to-peak value of these signals will vary as a function of the variation in the intensity of the Isubject illumination. The expression peak-to-peak value of the video signals denotes the difference between the voltage representing picture blackand the voltage representing picture white. In such a system it is necessary to employ apparatus either manually or automatically operable to convert the video signals having variable peak-to-peak values to control.

It is an object of the invention, therefore, to

` provide an automatic gain control for a multistage electron multiplier whereby to effect the production of a signal voltage `having apeak-to-peak value not exceeding a predetermined value,lirre spective of a variation of the difference between the lowest and highest primary electron concentrations.

In accordance-with the invention, there is provided an electron multiplier having a plurality of secondary electron 'emissive electrodes between which there is produced a secondary electron current flow. In order to automatically control the gain of the multiplier there is provided means controlled by the secondary electron current flow to vary the emissive effectiveness `oi" one or more of the electrodes as a function of the magnitude of the secondary electron current flow.

In accordance with the illustrative embodiment of the invention a multistage electron multiplier is provided with electrode potentials derived from a voltage divider connected between the terminals of a source of voltage. A variation in the overall l multiplication ratio of the multiplier is obtained whereby the signal amplication is made to depart from linearity as desired by providing the voltage divider with impedance components of a high order of magnitude as compared to the impedance elements of prior art voltage dividers employed in connection with electron multipliers, In this manner, the electrode potentials are varied under the control of the interelectrode electron current ow so that the peak-to-peak value of the video signals developed in the output circuit of the multiplier is maintained at a substantially constari't value, irrespective of subject illumination intensitychanges. In other words thev developed signals are subjected to a contrast control which varies as a function of the intensity of subject illumination.

In accordance with an additional feature of the invention a provision is made to enable a device of this character Ato produce a contrast control of the developed signals at relatively low subject illumination intensities. This feature comprises the connection of two successive multiplier electrodes to the same or very nearly the 4same potential.

For a better understanding of the invention, together with otherand further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing:

Fig. 1 is a diagrammatic representation of a dissector tube embodiment of the invention;

i Fig. 2 is' a schematic wiring diagram showing the electrical characteristics of the pertinent components of a dissector tube drawn to an enlarged scale; and,

Fig. 3 is a graphical representation of the performance of a dissector tube-multiplier combination embodiment of the invention.

Having reference now to Fig. 1 of the drawing, there is illustrated a sectional View of a dissector tube provided with an evacuated envelope I I.

Mounted within and adjacent to one end ofl the envelope is a photoelectric cathode I2. Also, within the envelope and adjacent to the end opposite the cathode there is mounted a multistage electron multiplier housed within a metallic shielding anode I3. The anode is provided withf a recessed portion I4 at the end of which is formed a primary scanning aperture I5 which is disposed substantially centrally with respect to the cathode I2 and faces the cathode. An accelerating anode is provided in the form of an focusing eld within the tube envelope whereby.

an electron image is formed substantially in the plane of the anode I3. The tube also is providedA with horizontal and vertical scanning coils I9 and 2I, respectively. Each of these coils is energized by respective sources 22' and 23 of i appropriate saw-toothed wave form voltages.

In conjunction with the dissector tube there also is provided an optical system represented by a lens 24, whereby an optical image of a subject 25 is focused into the plane of the photoelectric cathode I2.

Referring now to Fig. 2 of the drawing, a more detailed description of the electrical connections to the photoelectric cathode I2 and the electron multiplier will be given. The multiplier which is housed within the anode shield I3 comprises a plurality of box-like electrodes such as 26, 21, 28 and 29. In the case of a so-called elevenstage multiplier the device includes ten of such box-like electrodes, the eleventh stage ordinarily being in the form of a plate such as 3l. A collecting electrode of suitable form such as a grid 32 is located between the tenth stage electrode 29 and the eleventh stage electrode 3|. lThe first stage electrode 26 is provided with a secondary scanning aperture 33 in alignment with the primary scanning aperture formed in the anode shield I3. They secondary aperture ordinarily is smaller than Vthe primary aperture, being of substantially the same size as the desired ele mental scanning area. Y

Suitable accelerating potentials are impressed upon the multiplier electrodes and the photoelectric cathode by appropriate connections to taps on a voltage divider'which, as illustrated, comprises the series connection of resistors 34 to 44, inclusive. A source of relatively high potential, such as a battery 45, is connected to the terminals of the voltage divider. The negative terminal of the battery 45 is connected to the photoelectric cathode I2. The junction point between resistors 34 and 35 is connected to the shielding anode I3 and also, by means of an internal connection such as represented by a conductor 46, to the rst stage multiplier electrode 26. Thus, the anode shield and the rst stage electrode are operated at the same positive potential with respect to the cathode I2. Similarly, the electrodes comprising multiplier stages 2 to 8 inclusive, the latter of 'which comprises the electrode 21, are connected respectively to other taps on the voltage divider in a manner to impress increasingly higher positive potentials on succeeding multiplier electrodes. The ninth stage electrode 28 is connected to an adjustable tap 46' on the resistor 42. The tenth stage electrode 29 is connected to thejunction point between resistors 42 and'43 of the voltage divider. The eleventh stage electrode 3| is connected to the junction point between resistors 43 and 44. The other terminal of the resistor 44 is connected to the grounded positive terminal of the battery 45 and also to one terminal of an output resistor'41. The other terminal of the output resistor is connected to the collecting electrode 32 of the multiplier and also to one of a pair of output circuit terminals 48 of which the other terminal is connected to ground.

Referring now to the operation of the i1lus trated embodiment of this invention, assume for the moment that the adjustable tap 46 provided for the voltage divider resistor 42 is at the center of this resistor. 'I'he voltage divider resistors 35 to 44 inclusive', in accordance with this invention, are each of Vthe order of 200.000 ohms and the resistor 34 is of the order of 300,000 ohm's. The battery 45 is of the order of 2,500 volts, since it is desired to provide the multiplier electrodes, the anode and cathode with substantially the same nominal values of potential as in the prior art arrangements, which are of the order of 200 volts. With no electron current flowing in the multiplier, the circulating current through the voltage divider is'of the orderof .001 ampere. If it be assumed that, in order to develop the desired video signal output voltage, a maximum current ow of .0002 ampere is required, it is seen that this electron current is approximately 20% of the circulating current in the voltage divider as com- 1% of thecirculating current in a voltage divider -high intensity is admitted to the multiplier. veach succeeding stage of the multiplier there is a tendency for the production of a substantially Vincreased'electron current flow. By reason of .by an equal increase Idivider. f l

according to prior art arrangements. It is readily 'apparent that the voltage developed inv any one of the voltage divider resistors is much more greatly iniiuenced by variations in the electron current flow in the multiplier.

When the electron current ow in the multilplier is of a relatively low order of magnitude in Vresponse to a relatively small number of primary electrons admitted to the multiplier, the acceler- .ating potentials applied to the multiplier electrodes from the voltage divider resistors are substantially at their nominal values. Such a condition is illustrated by the straight line curve 49 of Fig. 3. This figure is an approximate graphical representation of the voltages impressed upon Athe multiplier electrodes, the cathode and anode A.of the dissector tube. These electrodes are indicated in the drawing, the cathode being desig- ,nated by the letter 7c, the anode by the letter a, and the multiplier electrodes by the numerals 'I to Il.

Vertical projections from any of the designatedelectrodes intersect the curve 49 at points which may be horizontally projected to determine the voltage differences between the ,electrodesq 'I'he curve 49 is seen to have two slightly diierent slopes indicating that the anode-to-cathode potential nominally is somewhat greater than the nominal potential diierences between any other two of the multiplier electrodes.

It is seen from the curve 49 that, at relatively 'low electron concentrations in the multiplier, the

nominal potential diierence between electrodes l and 2 is represented by the letter A. Similarly,

the potential difference between electrodes 9 and `I is represented by the letter B. It is seen that the differences A and B are substantially the same and also it is evident that this is true of any other two of the multiplier electrodes so long as the conditions are such that the curve 49 represents the operating characteristic of the multiplier. In a device in accordance with the instant invention the curve 49 represents the operating characteristic of the multiplier only at relatively low primary electron concentrations in the multhe inherent multiplying characteristic of such a device the electron current flow in the higher -stages such as between electrodes 21 and 28 tends to become relatively great. Since such a current ows in shunt with the corresponding voltage .divider resistor, such as the left hand portion of .the resistor 42, the voltage developed across this portion of the resistor is considerably less than in the case of a relatively low electron concentration Ain the multiplier.

Conversely, in the lower multiplier stages the tendency is for the interelectrode potentials to increase. This should be evidentfrom the fact that the voltage derived from the battery 45, which is assumed to be a source having a relatively good voltage regulation, does not change. Therefore, a decrease in voltage in one portionof the divider `must be compensated in another portion of the Referring again to Fig. 3, the curve illustrates an approximate graphical representation of the voltage distribution in the multiplier under conditions of a relatively high electron concentration. It is seen that, with respect to the electrodes of stages 1 and 2, the voltage diierence indicated by A is considerably greater than the corresponding voltage represented by A when the voltage distribution is in accordance with the curve 49. Likewise, at the other end of the multiplier, the electrode potential between stages 9 and 10 is represented, in the case under consideration, by B' which is seen to be considerably less than that represented by B in the case previously considered.

Thus, it is apparent that, as the primary electron concentrations in the multiplier increase, the interelectrode voltages available for accelerating purposes in the higher multiplier stages are decreased materially. It is true that, at the same time, the accelerating voltages in the lower stages of the multiplier are increased, but, it will be demonstrated presently that the lower multiplier stages have less effect on the final output signal Avoltages than the higher multiplier stages.

- The multiplication ratio of a given electron multiplier stage is the ratio between the number of impinging electrons and the number of secondary electrons emitted by the surface of the electrode. The number of secondary electrons emitted for a given number of impinging electrons is, within practical working limits, in direct proportion to the force with which the impinging electrons are made to strike the secondary emissive surface. This force is proportional to the magnitude of the voltage difference between the impinging electron source and the multiplier electrode. There, thus, is produced an increased number of secondary electrons by an increase of the accelerating voltage. Consequently, the multiplication ratio of a multiplier stage is increased by an increase of the accelerating voltage. However, such an increased multiplication ratio is realized only when all of the secondary electrons can be removed from the vicinity of the secondary V e'missive surface of the electrode. The secondary electron removal is effected by the accelerating voltage impressed between the electrode under consideration and the succeeding multiplier electrode. If this latter accelerating potential is insuiiicient to effect the removal of all of the secondary electrons a. space charge results and the multiplier stage in which the secondary electrons originate will exhibit an apparent multiplication ratio which is somewhat lower than the theoretically realizable ratio.

l.It is this phenomenon that occurs in the higher multiplier stages of the illustrated embodiment of the present invention. The interelectrode accelerating potential is decreased in the manner described so that a space charge and the resulting decrease in the multiplication ratios of one or more of these higher stages occurs. It is apparent that, for a given relatively low primary electron concentration, it is possible for only the eleventh multiplier stage to be so affected by space charge as to decrease its multiplication ratio. However, for a greater primary electron concentration the space charge phenomenon and the accompanying decrease of multiplication ratio may occur as early as the tenth multiplier stage, for example. Consequently, it is seen that the only effect of increased electron concentrations, resulting from increased multiplication ratios in the lower multiplier stages, is that space charge conditions will occur at a lower numbered one of the higher multiplier stages than in a conventionally operated multiplier. Therefore, it is evident that an increase in the multiplication ratio of the lower stages, resulting from the multiplier operation in accordance with this invention, merely increases the effectiveness of the automatic gain control. It follows, therefore, that there is eected a decrease in the overall multiplication ratio of the multiplier.

It has been found that, when operating an electron multiplier from a voltage supply derived from a voltage divider consisting of impedance components of the relatively high order of magnitude specified, the effective accelerating voltage between the ninth and tenth stages comprising electrodes 28 and 29 approaches the limiting value of zero at relatively high primary electron concentrations. Consequently, it is seen that there is effected substantially little or no electron multiplication in the tenth and eleventh stages, thereby, in effect, disabling these stages. `The video signal voltages developed in the output resistor 41 thus are no longer linear with respect to the primary electron concentration derived from different elemental areas of the photoelectrode cathode I2 where the illumination contrast between these areas is. of a relatively high order of magnitude such as will result from a relatively high intensity of subject illumination, for example. In this manner the peak-to-peak variations of the video signal voltages may be confined to a predetermined range of values, irrespective of the fluctuations in the subject illumination intensity.

It also has been found that, at relatively low values of subject illumination intensity, an additional improvement in operation may be secured by connecting together at least two of the higher multiplier electrodes so as to be operated at all times at the same potential. Using the illustrated embodiment of the invention as an example, this result may be obtained by moving the potentiometer contact 46 to the extreme right hand end of the resistor 42 so that itis connected to the junction point between this resistor and the resistor 43. In such a case the ninth and tenth stage electrodes 28 and 29 respectively, will have impressed thereon the same value of voltage. In like mannen-according to the illustrated form of the invention, multiplier stages 8 and 9 consisting of electrodes 21 and 28 may be operated at the same potential by moving the potentiometer contact 46 to the extreme left hand end 0f the resistor 42. It has been found that. while the latter type of connection produces improved results over those obtainable with prior art devices, the optimum performance of the multiplier is obtained by operating electrodes 28 and 29 at substantially the same potential.

It is obvious, of course, that intermediate positions of the potentiometer contact 46', either adjacent the left hand or the right hand ends of the resistor 42, also will produce substantially improved results. Accordingly, the disclosure of the particular type of resistor 42 with the adjustable contact 46' is intended merely to illustrate the general character of one embodiment of the invention and at the same time to indicate the flexibility `inherent in a device of this character. Consequently, it is not contemplated that the particular means for carrying the invention into practice described herein constitutes a limitation on the scope of the invention. It should Vbe apparent that the improved results produced by a multiplier operated in accordance with the invention are derived essentially by the use of inter-electrode accelerating voltage supplies which have relatively poor voltage regulation under variable load conditions. v

. 1 While there has been described what, at presmodications as fall within the true spirit and scope of the invention.

What is claimed is: i.

1.`-In a television system, apparatus for generating video signals comprising, a photoelectric electrode for producing a plurality of streams of primary electrons having limiting `value s of electron concentrations varying in accordance with the average intensity of light projected onto the electrode and representing a television subject. means including an electron multiplier located in the path of saidprimary electron streams for converting said streams into a video signal voltagehaving a peak-to-peak value not exceeding a predetermined value, said multiplier including a plurality of secondary electron'emissive electrodes, means disposed between said photoelectriclectrode and said multiplier for admitting said'primary electron streams successively to said multiplier, a source of accelerating voltage for said multiplier, a voltage divider connected to said voltage source and having xedand adjustable connections, means including said fixed connections for impressing predetermined fixed. accelerating voltages on certain ones of said multiplier electrodes, and means including.L an adjustable one "of said voltage divider connections for impressing on another one of said multiplier electrodes one of said fixed accelerating voltages.

2..`In a television system, apparatus for generating video signals comprisingy'a photoelectric electrode for producing aplurality of streams of primary electrons having limiting values of electroni concentrations varying in accordance with the average intensity of light projected onto the electrode and representing a televisionsubject, means including an electron multiplier located in the path of said primary electron streams for converting--said streams into a video signal voltage having a peak-to-peak valuefnot exceeding a predetermined value, said multiplier including a plurality of secondary electron emisslve'elec trodes, means disposed between said photoelectric`.electr0de and said multiplier for admitting said primary electron streams successively to'said multiplier, a source of accelerating voltage for saidmultiplier, a, voltage divider connected to said voltage source and having xed and adjustable connections, means including said xed connections for impressing predeterminedxed accelerating voltages on certain ones of said`multiplier electrodes, and means including an adjustable one of said voltage divider .connectionsfor '1m1- pressing on another one of said multiplier electrodes the same accelerating voltageas the iixed accelerating voltage impressed on a preceeding one of said certain multiplier electrodes. 3. In a television system, apparatus for generating video signals comprising, a photoelectric electrode for producing a pluralityof streams of primary electrons having limiting values of 'electron concentrations varying in" accordance with 9 the average intensity of light projected onto the electrode and representing a television subject, means including an electron multiplier located in the path of said primary electron streams for converting said streams into a video signal voltage having a peak-to-peak value not exceeding a predetermined value, said multiplier including a plurality of secondary electron emissive electrodes, means disposed lbetween said photoelectric electrode and said multiplier for admitting said primary electron streams successively to said multiplier, a source of accelerating voltage for said multiplier, a, Voltage divider connectedV to said voltage source and having xed and adjustable connections, means including said fixed connections for impressing predetermined fixed accelerating voltages on certain ones of said multiplier electrodes, and means including an adjustable one of said Voltage divider connections for impressing on another one of said multiplier electrodes the same accelerating Voltage as the fixed acceler- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,157,585 Zworykin et al. May 9, 1939 2,198,233 Snyder, Jr. Apr. 23, 1940 2,205,072 Skellet June 18, 1940 2,256,523 Lubszynski et a1. Sept. 23, 1941 2,290,775 Synder, Jr. July 21, 1942 2,291,577 Farnsworth July 28, 1942 2,310,883 Thom Feb. 9, 1943 FOREIGN PATENTS Number Country Date 502,686 Great Britain Mar. 22, 1939 

